Your users can use Sugar to send email messages from their own email accounts but they cannot use Sugar to check their email inboxes. Instead, they can store one-off email messages in Sugar when they need to. This email archiving is most commonly done via a connector like the Outlook Plug-in or by using the Sugar Email Archiving service (a.k.a. "SNIP"), which is basically a special email address that is used to send emails to your Sugar db.